1. Field of the Invention
The present invention relates to a polarization converter that has a dielectric waveguide and converts a polarization state of an electromagnetic wave to another polarization state.
2. Description of the Related Art
Currently, polarization converters are used in various applications. For example, a polarization converter is also used in a thermally-assisted magnetic head in which magnetic recording is assisted by heat.
JP 2010-088110 A discloses a polarization converter that uses a dielectric waveguide (see also FIGS. 1 and 2 of the present application). This polarization converter 90 has core parts 95a, 95b, 95c and a cladding part (not illustrated) covering around the core parts 95a, 95b, 95c. The core parts 95a, 95b, 95c have a substantially cuboidal shape, and a portion of the core part 95c is obliquely cut off in such a manner that an inclined surface 96 appears. The portion 95c (referred to as the asymmetric core part in the following) having the inclined surface 96 is a conversion part that converts the polarization state of an electromagnetic wave. An electromagnetic wave is incident onto the incident side core part 95a, passes through the asymmetric core part 95c, and is emitted from the emitting side core part 95b. 
When the electromagnetic wave is incident onto the asymmetric core part 95c, two polarization modes in diagonal directions that are orthogonal to each other are generated. Because of the asymmetry of a cross-sectional shape of the asymmetric core part 95c, a propagation constant (wave number) β1 of an electromagnetic wave of a first mode and a propagation constant (wave number) β2 of an electromagnetic wave of a second mode are different. Because of the difference in the propagation constants, as the electromagnetic waves propagate, a phase difference between the electromagnetic waves of the two modes increases. As a result, as an electromagnetic wave propagates through the asymmetric core part 95c, the polarization state of the electromagnetic wave changes.
The larger the difference (β1−β2) in the propagation constants is, the larger the phase difference of the electromagnetic waves becomes. Therefore, in order to shorten a length (converter length) of the asymmetric core part 95c along a propagation direction of the electromagnetic wave, it is necessary to increase the difference in the propagation constants. In order to increase the difference in the propagation constants, it is sufficient to increase the asymmetry of the cross-sectional shape of the asymmetric core part 95c, that is, to reduce the cross-sectional area of the asymmetric core part 95c. However, in this case, at a boundary between the input side core part 95a and the asymmetric core part 95c, and at a boundary between the asymmetric core part 95c and the output side core part 95b, coupling efficiency of the electromagnetic wave decreases, and as a result, use efficiency of the electromagnetic wave decreases. The decrease in the coupling efficiency is caused by an increase in a difference between a light distribution of the asymmetric core part 95c and light distributions of the incident side and emitting side core parts 95a, 95b. 
As described above, it is difficult to increase the coupling efficiency of the electromagnetic wave while reducing the converter length. Therefore, a polarization converter is desired that allows the coupling efficiency to be improved, in particular allows the coupling efficiency to be improved without significantly increasing the converter length.